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/* 2009 for Newlib: Sun's s_ilogb.c converted to be s_logb.c. */ /* @(#)s_ilogb.c 5.1 93/09/24 */ /* * ==================================================== * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. * * Developed at SunPro, a Sun Microsystems, Inc. business. * Permission to use, copy, modify, and distribute this * software is freely granted, provided that this notice * is preserved. * ==================================================== */ /* FUNCTION <<logb>>, <<logbf>>--get exponent of floating-point number INDEX logb INDEX logbf ANSI_SYNOPSIS #include <math.h> double logb(double <[x]>); float logbf(float <[x]>); DESCRIPTION The <<logb>> functions extract the exponent of <[x]>, as a signed integer value in floating-point format. If <[x]> is subnormal it is treated as though it were normalized; thus, for positive finite <[x]>, @ifnottex 1 <= (<[x]> * FLT_RADIX to the power (-logb(<[x]>))) < FLT_RADIX. @end ifnottex @tex $1 \leq ( x \cdot FLT\_RADIX ^ {-logb(x)} ) < FLT\_RADIX$. @end tex A domain error may occur if the argument is zero. In this floating-point implementation, FLT_RADIX is 2. Which also means that for finite <[x]>, <<logb>>(<[x]>) = <<floor>>(<<log2>>(<<fabs>>(<[x]>))). All nonzero, normal numbers can be described as @ifnottex <[m]> * 2**<[p]>, where 1.0 <= <[m]> < 2.0. @end ifnottex @tex $m \cdot 2^p$, where $1.0 \leq m < 2.0$. @end tex The <<logb>> functions examine the argument <[x]>, and return <[p]>. The <<frexp>> functions are similar to the <<logb>> functions, but returning <[m]> adjusted to the interval [.5, 1) or 0, and <[p]>+1. RETURNS @comment Formatting note: "$@" forces a new line When <[x]> is:@* +inf or -inf, +inf is returned;@* NaN, NaN is returned;@* 0, -inf is returned, and the divide-by-zero exception is raised;@* otherwise, the <<logb>> functions return the signed exponent of <[x]>. PORTABILITY ANSI C, POSIX SEEALSO frexp, ilogb */ /* double logb(double x) * return the binary exponent of non-zero x * logb(0) = -inf, raise divide-by-zero floating point exception * logb(+inf|-inf) = +inf (no signal is raised) * logb(NaN) = NaN (no signal is raised) * Per C99 recommendation, a NaN argument is returned unchanged. */ #include "fdlibm.h" #ifndef _DOUBLE_IS_32BITS double #ifdef __STDC__ logb(double x) #else logb(x) double x; #endif { __int32_t hx,lx,ix; EXTRACT_WORDS(hx,lx,x); hx &= 0x7fffffff; /* high |x| */ if(hx<0x00100000) { /* 0 or subnormal */ if((hx|lx)==0) { double xx; /* arg==0: return -inf and raise divide-by-zero exception */ INSERT_WORDS(xx,hx,lx); /* +0.0 */ return -1./xx; /* logb(0) = -inf */ } else /* subnormal x */ if(hx==0) { for (ix = -1043; lx>0; lx<<=1) ix -=1; } else { for (ix = -1022,hx<<=11; hx>0; hx<<=1) ix -=1; } return (double) ix; } else if (hx<0x7ff00000) return (hx>>20)-1023; /* normal # */ else if (hx>0x7ff00000 || lx) return x; /* x==NaN */ else return HUGE_VAL; /* x==inf (+ or -) */ } #endif /* _DOUBLE_IS_32BITS */